The present invention concerns receivers capable of determining pseudoranges to orbiting satellites and, in particular, concerns such receivers as find application in global positioning satellite systems (GPS).
GPS receivers normally determine their position by computing relative times of arrival of signals transmitted simultaneously from a multiplicity of GPS (or NAVSTAR) satellites. These satellites transmit, as part of their message, both satellite positioning data as well as data on clock timing, so-called xe2x80x9cephemerisxe2x80x9d data. The process of searching for and acquiring GPS signals, reading the ephemeris data for a multiplicity of satellites and computing the location of the receiver from this data is time consuming, often requiring several minutes. In many cases, this lengthy processing time is unacceptable and, furthermore, greatly limits battery life in micro-miniaturized portable applications.
Another limitation of current GPS receivers is that their operation is limited to situations in which multiple satellites are clearly in view, without obstructions, and where a good quality antenna is properly positioned to receive such signals. As such, they normally are unusable in portable, body mounted applications; in areas where there is significant foliage or building blockage (e.g., urban canyons); and in in-building applications.
There are two principal functions of GPS receiving systems: (1) computation of the pseudoranges to the various GPS satellites, and (2) computation of the position of the receiving platform using these pseudoranges and satellite timing and ephemeris data. The pseudoranges are simply the time delays measured between the received signal from each satellite and a local clock. The satellite ephemeris and timing data is extracted from the GPS signal once it is acquired and tracked. As stated above, collecting this information normally takes a relatively long time (30 seconds to several minutes) and must be accomplished with a good received signal level in order to achieve low error rates.
Virtually all known GPS receivers utilize correlation methods to compute pseudoranges. GPS signals contain high rate repetitive signals called pseudorandom (PN) sequences. The codes available for civilian applications are called C/A codes and have a binary phase-reversal rate, or xe2x80x9cchippingxe2x80x9d rate, of 1.023 MHz and a repetition period of 1023 chips for a code period of 1 msec. The code sequences belong to a family known as Gold codes. Each GPS satellite broadcasts a signal with a unique Gold code.
For a signal received from a given GPS satellite, following a downconversion process to baseband, a correlation receiver multiplies the received signal by a stored replica of the appropriate Gold code contained within its local memory, and then integrates, or lowpass filters, the product in order to obtain an indication of the presence of the signal. This process is termed a xe2x80x9ccorrelationxe2x80x9d operation. By sequentially adjusting the relative timing of this stored replica relative to the received signal, and observing the correlation output, the receiver can determine the time delay between the received signal and a local clock. The initial determination of the presence of such an output is termed xe2x80x9cacquisition.xe2x80x9d Once acquisition occurs, the process enters the xe2x80x9ctrackingxe2x80x9d phase in which the timing of the local reference is adjusted in small amounts in order to maintain a high correlation output. The correlation output during the tracking phase may be viewed as the GPS signal with the pseudorandom code removed, or, in common terminology, xe2x80x9cdespread.xe2x80x9d This signal is narrow band, with bandwidth commensurate with a 50 bit per second binary phase shift keyed data signal which is superimposed on the GPS waveform.
The correlation acquisition process is very time consuming, especially if received signals are weak. To improve acquisition time, most GPS receivers utilize a multiplicity of correlators (up to 12 typically) which allows a parallel search for correlation peaks.
Some prior GPS receivers have used FFT techniques to determine the Doppler frequency of the received GPS signal. These receivers utilize conventional correlation operations to despread the GPS signal and provide a narrow band signal with bandwidth typically in the range of 10 kHz to 30 kHz. The resulting narrow band signal is then Fourier analyzed using FFT algorithms to determine the carrier frequency. The determination of such a carrier simultaneously provides an indication that the local PN reference is adjusted to the correct phase of the received signal and provides an accurate measurement of carrier frequency. This frequency may then be utilized in the tracking operation of the receivers.
U.S. Pat. No. 5,420,592 to Johnson discusses the use of FFT algorithms to compute pseudoranges at a central processing location rather than at a mobile unit. According to that method, a snapshot of data is collected by a GPS receiver and then transmitted over a data link to a remote receiver where it undergoes FFT processing. However, the method disclosed therein computes only a single forward and inverse Fast Fourier Transform (corresponding to four PN periods) to perform the set of correlations.
One embodiment of the present invention provides a global positioning system (GPS) receiver having first circuitry for receiving and processing pseudorandom sequences transmitted by a number of GPS satellites. The first circuitry is configured to perform conventional correlation operations on the received pseudorandom sequences to determine pseudoranges from the GPS receiver to the GPS satellites. The GPS receiver also includes second circuitry coupled to the first circuitry. The second circuitry is configured to receive and process the pseudorandom sequences during blockage conditions. The second circuitry processes the pseudorandom sequences by digitizing and storing a predetermined record length of the received sequences and then performs fast convolution operations on the stored data to determine the pseudoranges.
In one embodiment, the GPS receiver has a common antenna for receiving GPS signals from in view satellites; and a common downconverter for reducing the RF frequency of the received GPS signals to an intermediate frequency (IF). The IF signals are then split into two signal paths. A first of the signal paths provides for conventional GPS signal processing using correlation operations to calculate the pseudoranges. During blockage conditions, the IF signal is passed to the second signal path wherein the IF signals are digitized and stored in memory for later processing in the receiver. This later processing is accomplished using a programmable digital signal processor which executes the instructions necessary to perform fast convolution operations on the sampled IF GPS signals to provide the pseudoranges.
In yet another embodiment of the present invention, the GPS receiver has a common antenna for receiving GPS signals from in view satellites and a switch for choosing between two signal paths. A first of the signal paths provides for conventional GPS signal processing, wherein pseudoranges are calculated using correlation operations. During blockage conditions, a second signal path is used wherein the signals are digitized and stored in memory for later processing. This later processing is accomplished using fast convolution operations on the sampled GPS signals to provide the pseudoranges.
A further embodiment of the present invention provides a GPS receiver with a common antenna for receiving GPS signals from in view satellites and a common downconverter and digitizer. Sampled GPS signals received from the in view satellites are provided to a first signal path for conventional correlation processing to determine pseudoranges. During blockage conditions, the sampled GPS signals are provided to a second signal path for processing using fast convolution operations to determine the pseudoranges. The two signal paths may be provided by separate circuitry or by common circuitry executing computer readable instructions appropriate for the given reception conditions.
An additional embodiment of the present invention provides a method for determining the position of a remote GPS receiver by storing GPS satellite information, including Doppler, in the remote unit. During blockage conditions, the remote unit uses this information and sampled GPS signals from in view satellites to subsequently compute pseudoranges to the satellites using fast convolution operations. The computed pseudoranges may then used to determine the position of the remote unit. The position determination can occur at the remote unit or at a basestation. Where the position determination is performed at a basestation, the remote unit transmits the pseudoranges to the basestation via a data link.